The present invention relates to structured packing and to methods for installing such packing in an exchange column to provide increased capacity. The structured packing has particular application in cryogenic air separation processes, although it also may be used in other heat and/or mass transfer processes that can utilize structured packing.
The term, “column” (or “exchange column”) as used herein, means a distillation or fractionation column or zone, i.e., a column or zone wherein liquid and vapor phases are counter currently contacted to effect separation of a fluid mixture, such as by contacting of the vapor and liquid phases on packing elements or on a series of vertically-spaced trays or plates mounted within the column.
The term “column section” (or “section”) means a zone in a column filling the column diameter. The top or bottom of a particular section or zone ends at the liquid and vapor distributors respectively.
The term “packing” means solid or hollow bodies of predetermined size, shape, and configuration used as column internals to provide surface area for the liquid to allow mass transfer at the liquid-vapor interface during countercurrent flow of two phases. Two broad classes of packings are “random” and “structured”.
“Random packing” means packing wherein individual members do not have any particular orientation relative to each other or to the column axis. Random packings are small, hollow structures with large surface area per unit volume that are loaded at random into a column.
“Structured packing” means packing wherein individual members have specific orientation relative to each other and to the column axis. Structured packings usually are made of thin metal foil, expanded metal or woven wire screen stacked in layers or as spiral windings.
In processes such as distillation or direct contact cooling, it is advantageous to use structured packing to promote heat and mass transfer between counter-flowing liquid and vapor streams. Structured packing, when compared with random packing or trays, offers the benefits of higher efficiency for heat and mass transfer with lower pressure drop. It also has more predictable performance than random packing.
Cryogenic separation of air is carried out by passing liquid and vapor in countercurrent contact through a distillation column. A vapor phase of the mixture ascends with an ever increasing concentration of the more volatile components (e.g., nitrogen) while a liquid phase of the mixture descends with an ever increasing concentration of the less volatile components (e.g., oxygen). Various packings or trays may be used to bring the liquid and gaseous phases of the mixture into contact to accomplish mass transfer between the phases.
The most commonly used structured packing consists of corrugated sheets of metal or plastic foils (or corrugated mesh cloths) stacked vertically. These foils may have various forms of apertures and/or surface texture features aimed at improving the heat and mass transfer efficiency. An example of this type of structured packing is disclosed in U.S. Pat. No. 4,296,050 (Meier). It also is well-known in the prior art that mesh type packing helps spread liquid efficiently and gives good mass transfer performance, but mesh type packing is much more expensive than most foil type packing.
It is very desirable to increase the capacity of structured packing. This enables the use of less structured packing for any given separation, thus reducing the cost of carrying out the separation.
Usually the capacity of structured packing is limited by flooding. Mass transfer flooding, which is the premature degradation in mass transfer performance prior to the onset of hydraulic flooding, occurs when the mass transfer efficiency of the column starts deteriorating rapidly with the increase of vapor and/or liquid flow in the column. Hydraulic flooding occurs when the pressure drop across the packing bed starts increasing rapidly with the increase of vapor and/or liquid flow.
It is known from the prior art that the capacity of structured packing can be increased by modifying the edges of individual packing sheets. Typical modifications include reduced crimp heights, changed corrugation angle, serrations, etc., which modifications are typically made at the bottom of all sheets or at the top and bottom of alternating sheets. Examples of such modifications are disclosed in U.S. Pat. No. 5,632,934 (Billingham, et al.) and U.S. Pat. No. 6,101,841 (Billlingham, et al.). Other modifications include S-shaped corrugations on both ends of every sheet, such as those disclosed in EP 0 858 366 B1 and International Application WO 97/16247. All such modifications are made in such a way that during operation the pressure drop in the transitions is reduced. Operation of a packed column at a pressure drop greater than 0.7 inch water per foot is taught in U.S. Pat. No. 5,921,109 (Billingham, et al) and U.S. Pat. No. 6,212,907 B1 (Billingham, et al.). These patents cover cases wherein only the bottoms of the packing sheets are modified, and cases wherein both the tops and bottoms of the packing sheets are modified.
Although these prior art modifications may improve the capacity of structured packing, those designs still suffer from a problem of uncontrolled and random gaps between adjacent layers of structured packing, which gaps are a result of the manufacturing methods used for conventional structured packing sheets. The uncontrolled gaps between adjacent layers are due to a less than perfect alignment of the individual structured packing elements or sheets within the layers. This can lead to performance degradation in terms of both mass transfer and pressure drop, especially at high flow rates.
It is desired to have improved structured packing sheets or elements which significantly increase the capacity of structured packing.
It is further desired to have a structured packing which enables improved performance over that of conventional structured packing.
It is still further desired to have a structured packing for use in an exchange column having increased capacity enabling increased throughput before reaching flood conditions.
It is still further desired to have a structured packing that shows high performance characteristics for cryogenic applications, such as those used in air separation, and for other heat and/or mass transfer applications.
It is still further desired to have a structured packing of the corrugated type which overcomes many of the difficulties and disadvantages of the prior art to provide better and more advantageous results.
It is still further desired to have a process for cryogenic air separation which utilizes a structured packing that provides higher capacity than that of prior art structured packing.
It is still further desired to have a process for cryogenic air separation which may be carried out at increased capacity while avoiding flooding.
It also is desired to have a method of assembling and installing a section of structured packing in an exchange column which affords better performance than the prior art, and which also overcomes many of the difficulties and disadvantages of the prior art to provide better and more advantageous results.